18
trying(adjective, hard, difficult, complex)
18
unbearable(adjective, hard, difficult, complex)
17
substantial(adjective, importance, big, heavy, important)
16
pressing(adjective, hard, tough, difficult, big)
15
dangerous(adjective, help, bad, great, severe)
15
crucial(adjective, great, severe, extraordinary, big)
15
grand(adjective, importance, valuable, main, greatness)
15
great(adjective, importance, valuable, main, notable)
15
meaningful(adjective, good, great, severe, extraordinary)
15
sincere(noun, adjective, open, keen, warm)
14
acute(adjective, severe, harsh, heavy, important)
14
big(adjective, good, importance, heavy, important)
14
urgent(adjective, hard, tough, severe, difficult)
13
material(adjective, good, importance, big, heavy)
13
portentous(adjective, importance, valuable, main, notable)
12
considerable(adjective, importance, big, heavy, important)
12
key(adjective, importance, important, valuable, main)
12
sombre(adjective, solemn)
11
deep(adjective, keen, important, significant, thoughtful)
11
essential(adjective, severe, importance, big, important)
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APASmart Define. (2016). serious thesaurus. Retrieved September 23, 2026, from https://smartdefine.org/serious/thesaurus/synonyms
ChicagoSmart Define. 2016. "serious thesaurus" https://smartdefine.org/serious/thesaurus/synonyms (accessed September 23, 2026).
HarvardSmart Define 2016, serious thesaurus, Smart Define, viewed 23 September, 2026, <https://smartdefine.org/serious/thesaurus/synonyms>.
MLASmart Define. "serious thesaurus" 24 February 2016. Web. 23 September 2026. <https://smartdefine.org/serious/thesaurus/synonyms>